The goal of this R03 application is to determine in vivo roles of fascin1 in dendritic cell (DC)-based immunity against infection of Listeria monocytogenes (Lm). DCs, the most potent antigen presenting cells, play central roles in both innate and acquired immunity. When DCs encounter pathogens, they undergo terminal differentiation called maturation: they assemble numerous veil-like membrane protrusions, travel from the peripheral tissues to draining lymph nodes, and present antigens to naive T-cells. Fascin1 is a unique actin-bundling protein in DCs because it is greatly and specifically induced upon DC maturation. Fascin1 is absent in immature DCs or in other blood cells including macrophages, neutrophils and natural killer cells, suggesting a specific role for fascin1 in DC biology upon maturation. Indeed, we and others have demonstrated that fascin1 is critical for the maturation-associated functions including massive reorganization of the actin cytoskeleton, chemotactic migration of DCs into draining lymph nodes, T-cell activation and assembly of the immunological synapse. We have found, by characterizing fascin1 KO DCs, that fascin1 plays a novel role in a DC-mediated host defense system. It has been reported that DCs show a stronger intracellular killing activity toward Lm than do macrophages. This resistance to Lm is thought to be crucial for the DCs primary function of antigen presentation: DCs must survive infection at the periphery and then move to draining lymph nodes for transferring antigen information to T-cells. We found that fascin1 KO DCs displayed higher susceptibility to Lm-mediated killing. Importantly, DCs with high fascin1 expression was extremely resistant to Lm infection. We further found that fascin1 made Lm-encapsulated phagosomes more acidic, and increases autophagy flux. These results indicate that fascin1 increases lysosomal fusion of both phagosomes and autophagosomes for bacterial clearance, explaining why DCs (fascin1-positive) are more resistant than macrophages (fascin1-negative). However, a key question remains to be answered: Is fascin1 crucial for DC-based adaptive and innate immunity against Lm infection in vivo? We will examine, by using the Lm mouse model, whether fascin1 KO mice show reduced adaptive and innate immune responses to Lm infection. The proposal is likely to provide vital information that will help us develop new DC-based immunotherapy against bacterial infection. PUBLIC HEALTH RELEVANCE: The elucidation of the in vivo role of fascin1 in innate and adaptive immunity against Listeria monocytogenes (Lm) infection could have a transformative effect on our understanding of host defense toward intracellular pathogens. Fascin1-expressing dendritic cells (DCs), by controlling intracellular proliferation of Lm and possibly other pathogens, are expected to effectively transfer pathogen information to T-cells, thereby leading to clearance of bacterial pathogens. We suggest that fascin1-expressing dendritic cells (DCs) could thus be utilized in DC-based immunotherapy.